Comparison of Heparin Anti-Xa Activity From Central Venous Catheter Samples Using a 5 mL Syringe Flush or a Vacuum Tube Flush Versus Peripheral Vein Samples in ICU Patients

The goal is to investigate whether blood samples drawn from a central venous catheter can provide reliable measurements of unfractionated heparin (UFH) anti-Xa activity, compared to the standard method of peripheral vein puncture, in intensive care unit (ICU) patients receiving continuous intravenous UFH. To evaluate the reliability of central venous blood sampling, the study will compare anti-Xa activity levels obtained simultaneously from two different types of blood draws: one from a peripheral vein (reference method), and the other from the central line using one of two flushing techniques. The two central flushing techniques being studied are: * A 5 mL syringe flush performed over 5 seconds, followed by blood collection. * A vacuum tube flush that draws and discards 5 mL of blood, followed by blood collection. Each patient will undergo four pairs of simultaneous blood draws, usi